(770b) Co-Fermentation of Glucose and Xylose From Steam Pretreated Wheat Straw With Recombinant S. Cerevisiae Strains

Advanced bioethanol produced by fermentative conversion of lignocellulosic agricultural residues relies on efficient hydrolysis and fermentation, as well as extensive utilization of available substrates in order to obtain high yields and render the process economically viable. Another important aspect is high ethanol concentration in the fermentation broth, as this reduces the energy demand. Obtaining a high ethanol concentration requires high water insoluble solids (WIS) content in simultaneous saccharification and fermentation (SSF) or in the enzymatic hydrolysis in separate hydrolysis and fermentation (SHF). This usually has an adverse effect on ethanol yield, due to the increased inhibition and poor mass transfer. Opting for either configuration is essentially a trade-off between optimal temperatures and inhibitory glucose concentrations on the one hand (SHF) and sub-optimal temperatures and ethanol-inhibited cellulolysis on the other (SSF). However, the ethanol yield and concentration can also be improved by efficient co-fermentation of glucose and xylose; especially from agricultural residues, such as wheat straw, due to the high amount of hemicelluloses in these materials.

In the present study, SSF and SHF of steam pretreated wheat straw was performed in a variety of process configurations in 2 to 30 L scale. Two different recombinant S. cerevisiae strains, which has the ability to co-ferment glucose and xylose, and two steam-pretreated wheat straw slurries, one pretreated at mild conditions and one pretreated at more sever conditions, were utilized. Substrate feeding and enzyme dosage were varied in order to maximize the ethanol yield. The highest overall ethanol yield achieved, based on the amount of glucose and xylose present in the material fed to the system, was 92% of theoretical for the milder pretreated straw and 84% of theoretical for the more severe pretreated straw. The ethanol concentration was 45.5 and 41.9 g/L, respectively. In both cases the xylose consumption was higher than 90% and the xylitol produced was 1.8 and 3.3 g/L respectively.